To reproduce:

1. Download attached project "20190116.zip" and open in Unity
2. Open "SampleScene" scene
3. Build for iOS
4. Deploy the Xcode project to iOS Device

Expected results: Unity Ads initializes and works correctly on the iOS device
Actual results: Unity Ads doesn't work due to MissingMethodException: Default constructor not found for type UnityEngine.Advertisements.Platform

Notes:
- This issue appears only on iOS
- On Android (Mono and IL2CPP) everything works as expected
- Stack Trace is shared in Comments section

Tested with:
- iPhone 7 Plus, iOS: 12.0 - Reproduced
- iPhone X, iOS: 11.3.1 - Reproduced

Reproduced on Package version 2.3.2
Does not reproduced on Package version 2.3.1
Regression on Package version 2.3.2

Reproduced on Unity 2018.3.3f1, 2019.1.0a14 and 2019.2.0a2